Irritant Gases
Acids & Corrosives · Acids & Corrosives overview
Irritant gases — hydrogen chloride, sulphur dioxide, nitrogen dioxide, chlorine, ammonia, ozone, hydrogen fluoride and the lower aldehydes — are the gaseous co-products of many acid, corrosive and oxidising processes in UK industry. They act on the eyes, nose, throat and airways within seconds of inhalation and, for the poorly water-soluble gases such as NO2 and ozone, can cause delayed pulmonary effects hours later. Their workplace exposure is controlled under the Control of Substances Hazardous to Health (COSHH) Regulations 2002 against the substance-specific Workplace Exposure Limits in HSE EH40.
What an irritant gas does to the airway
Highly water-soluble gases — ammonia, HCl, SO2 — are absorbed in the upper airway, producing immediate burning of the nose, throat and eyes that drives operators out of the exposure. Poorly water-soluble gases — NO2, ozone, phosgene — pass the upper airway with little warning and reach the deep lung, where they can cause delayed chemical pneumonitis and pulmonary oedema hours after a short exposure. The exposure-limit strategy reflects this: most irritants have both an 8-hour TWA and a 15-minute short-term limit, and several are tagged with a 'Sen' notation for sensitisation potential.
Where irritant gases are generated
Irritant gases arise as the intended or unintended product of acid, corrosive and oxidising processes across UK industry.
- HCl from steel pickling, galvanising preparation and reactive chlorinations.
- SO2 from sulphuric acid handling, lead-acid battery formation and combustion plant.
- NO2 from nitric passivation, mixed-acid pickling and precious-metal refining.
- Chlorine from electrolysis, water treatment and hypochlorite/acid contact.
- Ammonia from refrigeration plant, electroplating and chemical processing.
- Ozone from disinfection plant, UV and corona-discharge processes.
- Hydrogen fluoride from glass etching, mixed-acid pickling and aluminium processing.
- Lower aldehydes from heat-treatment, sterilisation and embalming processes.
Why irritant gas exposure matters
Irritant gases are responsible for many of the acute hospitalisations from chemical exposure recorded in UK workplaces. They also drive a substantial proportion of occupational asthma cases — chlorine, formaldehyde, glutaraldehyde and isocyanates (where they appear in irritant-gas form) are all recognised triggers. Even sub-WEL chronic exposure is associated with chronic upper-airway and ocular irritation, sleep disturbance and aggravation of pre-existing respiratory disease.
Because several irritants have low odour thresholds, operators may believe they have 'got used to' a smell that is in fact above the WEL. Quantitative sampling reliably overturns this perception.
Monitoring irritant gases
Each gas is sampled by a substance-specific method. HCl, SO2 and HF are typically sampled on treated filters or sorbent tubes with IC analysis. NO2 is sampled with diffusive samplers or sorbent tubes for both the 8-hour TWA and the short-term limit. Chlorine and ozone are typically sampled with direct-reading electrochemical instruments backed by occasional sorbent-tube confirmation. Ammonia is sampled on a sorbent tube against the EH40 limit. Personal sampling on the most exposed operator drives the COSHH judgement; static instruments support plant-room and incident-zone monitoring.
Sampling strategy follows BS EN 689, with explicit attention to the short-term limit during loading, dosing, maintenance and reactive peaks.
Controlling irritant gas exposure
Control follows the COSHH hierarchy: substitute or eliminate the gas-generating step where possible (citric for nitric passivation; sealed CIP for hypochlorite; closed transfer for ammonia and chlorine), enclose and lid the process, provide LEV designed for the worst-case gas — alkaline scrubbing for HCl, NO2 and chlorine — and scrub the discharge. Plant-room and pit ventilation must move the denser gases (chlorine, NO2) from low points to scrubbed extract.
Provide RPE rated for the worst credible release for tank cleaning, breakdown response and confined-space entry. Detector alarms in chlorine, ammonia and HF rooms back up the engineering controls.
When to review irritant gas exposure
Review on a change of chemistry, throughput, plant layout or LEV; on alarms, near-misses or operator symptoms; following enforcement or insurer review; and at a defined two- to three-year refresh. LEV serving irritant-gas processes requires Thorough Examination and Test at least every 14 months under COSHH Reg 9.
Frequently asked questions
Can operators rely on smell to detect irritant gases?
No. Several irritant gases (NO2, ozone) give little warning before reaching damaging concentrations, and operators acclimatise to gases such as ammonia and chlorine, losing the early-warning signal. Quantitative sampling and fixed detectors are needed.
Why are short-term EH40 limits important for irritants?
Because irritant exposures are usually short and intense — a tank top-up, a maintenance error, a reactive peak. The 15-minute short-term limit is set specifically to control these episodes, not the steady-state average.
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